The present invention relates to methods of producing toners for use in developing of electrophotographs in copying machines, printing machines and the like. And it relates more particularly to a method of producing a toner which method is high in production yield, which toner is stable in an electrostatic property and excellent in durability.
Ordinarily, an addition of an outer-additive to colored particles has conventionally been performed. An object of such an addition is to provide a toner to be obtained with flowability, to control an electrostatic property thereof and the like. Recently, an enhancement of a printing quality, particularly an enhancement of resolution, has strongly been required in an electrophotograph. Therefore, it is important to allow an outer-additive containing little aggregate to be firmly adhered to colored particles or to be partially embedded therein.
In order to allow the outer-additive containing little aggregate to be adhered to or partially embedded in the colored particles, a mixer with an agitating blade that rotates at a high speed has been used. Various types of conditions under which the outer-additive is added to the colored particles by using such a mixer have been studied.
Japanese Patent No. 2,921,174 discloses a technique in which, when an outer-additive is added to colored particles, a shear rate calculated on the basis of a tip speed of a blade in a mixer and a clearance between the blade and an inner wall of the mixer, and a mixing time of the outer-additive are allowed to be in a specified range. In this publication, it is disclosed that, by this technique, a toner for developing an electrostatic latent image that has a high frictional electrification property and good flowability, but does not generate a faulty transfer and an image defect can be obtained. Japanese Unexamined Patent Publication No. 120969/1995 discloses a technique in which, when an outer-additive is added to colored particles, a mixing apparatus comprising a mixing tank having a given capacity and an agitating blade which is rotatable such that it gives shear strength to particles is used while pressure inside a mixing vessel is reduced. In this publication, it is disclosed that, by this technique, a magnetic toner which has high flowability and a high electrostatic property and can perform development of an electrophotograph with an image quality of little fog and high density can be obtained. Further, Japanese Unexamined Patent Publication No. 319629/1998 discloses a technique in which, between a process of removing large-size particles and a process of adding an outer-additive, added is a process of premixing for crushing aggregated colored particles. In this publication, it is disclosed that, by this technique, it is possible to allow dispersion of an outer-additive in a toner to be uniform and also a toner which does not generate any thinning in an electrophotograph derived from insufficient flowability and fog of the electrophotograph derived from an insufficient frictional electrification property to be obtained.
The present inventors have examined these techniques in detail and found that said techniques are incapable of sufficiently crushing aggregated outer-additives and allowing the thus-crushed outer-additives to be firmly adhered to or partially embedded in colored particles. The present inventors have also found that, when a toner obtained by said techniques is used in development of an electrophotograph, problems that a white streak or fog is generated in the thus-produced electrophotograph still remain unsolved.
An object of the present invention is to provide a method for producing a toner which gives a high yield and generates a small quantity of aggregates after colored particles and an outer-additive are mixed with one another, in which the toner can form an image having little white streak and fog when it is used in development of an electrophotograph.
The present inventors have conducted an extensive study for attaining said object and, as a result, have found that, when colored particles and an outer-additive are mixed with one another, said object can be attained by allowing a flowing quantity of a gas sealing a shaft portion of an agitating blade of a mixer and a peripheral rate at a tip of the agitating blade to be values each in a specified range. The present invention has been accomplished on the basis of this finding.
Thus, according to the present invention, there is provided a method for producing a toner comprising a process of mixing colored particles and an outer-additive by a mixer with an agitating blade whose shaft portion is sealed by a gas having a flowing quantity of from 0.4 m3/hr to 3 m3/hr and which rotates at such a high speed as a peripheral speed at a tip thereof being from 20 m/sec to 50 m/sec. According to the present invention, the process of mixing the colored particles and the outer-additive by the mixer is denoted as a xe2x80x9cprocess P4xe2x80x9d.
It is preferable that large-size particles are removed in advance from the colored particles used in the process P4. According to the present invention, the colored particles before the large-size particles are removed therefrom are denoted as xe2x80x9cunrefined colored particlesxe2x80x9d and a process of obtaining the colored particles by removing the large-size particles from the unrefined colored particles is denoted as a xe2x80x9cprocess P3xe2x80x9d.
It is more preferable that the process P3 is a process of removing the large-size particles from the unrefined colored particles by providing the unrefined colored particles to a screen having an opening of from 5 xcexcm to 40 xcexcm and allowing them to pass therethrough by using air flow. The reason for such preference is that, when development of an electrophotograph is performed by using a toner obtained by such a process, an image defect such as filming or a blank area is not generated and the process itself is low in cost, high in yield and easy in setting operational conditions.
It is preferable that the unrefined colored particles to be used in the process P3 are first prepared in a state of a dispersion liquid containing them and, then, are separated from the dispersion liquid by using a solid-liquid separator and, thereafter, are dried. According to the present invention, the unrefined colored particles which have been separated from the dispersion liquid containing them by using the solid-liquid separator, but have yet to be dried, namely, the unrefined colored particles before being dried, are denoted as xe2x80x9cunrefined colored particles in a wet statexe2x80x9d, and a process of obtaining the unrefined colored particles in a wet state from the dispersion liquid of the unrefined colored particles by using the solid-liquid separator is denoted as xe2x80x9cprocess P1xe2x80x9d and, further, a process of drying said unrefined colored particles in a wet state to obtain the unrefined colored particles is denoted as xe2x80x9cprocess P2xe2x80x9d.
It is more preferable that, in the process P1, the solid-liquid separator that is to be used at the end of the process P1 comprises a porous metallic body as a filter material. The reason for such preference is that, by adopting such a process, it is possible to efficiently separate the unrefined colored particles in a wet state from the dispersion liquid of the unrefined colored particles and, in such a process, there is no need for such maintenance as to change filter materials or the like and also, in such a process, a running cost is low.
It is more preferable that the process P2 is a process that vacuum-dries the unrefined colored particles in a wet state by using a vacuum dryer with an agitating blade under a condition that a transfer ratio R is in a range of from 40 to 500. The unrefined colored particles that have been subjected to such a process are low in a content of the large-size particles and, when development of electrophotograph is performed by using the toner that has been subjected to such a process, image defects such as filming and fog are not generated; therefore, such a process is efficient. On this occasion, transfer ratio R=X*Y/Z, wherein X [m3/Hr] represents transfer flowing quantity of an agitating blade of a vacuum dryer; Y [Hr] represents a drying time; and Z [m3] represents a provided volume quantity of the unrefined colored particles in a wet state.